The Ethernet physical layer is the physical layer component of the Ethernet family of computer network standards.

The Ethernet physical layer evolved over a considerable time span and encompasses quite a few physical media interfaces and several magnitudes of speed. The speed ranges from 1 Mbit/s to 100 Gbit/s while the physical medium can range from bulky coaxial cable to twisted pair to optical fiber. In general, network protocol stack software will work similarly on all physical layers.

10 Gigabit Ethernet is becoming more popular in both enterprise and carrier networks, with 40 Gbit/s and 100 Gbit/s Ethernet ratified. Higher speeds are under development. Metcalfe now believes commercial applications using terabit Ethernet may occur by 2015 though he says existing Ethernet standards may have to be overthrown to reach terabit Ethernet.

Many Ethernet adapters and switch ports support multiple speeds, using autonegotiation to set the speed and duplex for the best values supported by both connected devices. If auto-negotiation fails, a multiple speed device will sense the speed used by its partner, but will assume half-duplex. A 10/100 Ethernet port supports 10BASE-T and 100BASE-TX. A 10/100/1000 Ethernet port supports 10BASE-T, 100BASE-TX, and 1000BASE-T.